Method and apparatus for the thermal treatment of alkali-containing pulverized raw material to be used in the manufacture of cement

ABSTRACT

A method and apparatus for the thermal treatment of alkali-containing pulverent raw material used in the manufacture of cement in which the material is preheated in a preheater and after sintering in a furnace is conveyed to a cooler. Alkali free exhaust air from the cooler is conveyed into the preheater for heating the material and the alkali-containing furnace exhaust gases are cooled by means of adding a portion of the pulverized raw material to them in a heat exchanger. The alkali salts are condensed and are separated off in a dust collecting chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is in the field of thermal treatment of alkali-containingpulverent raw material used in the manufacture of cement to achieveminimum heat losses and to produce calcined products of controlledalkali content despite variations in alkali content in the raw material.

2. DESCRIPTION OF THE PRIOR ART

It is known in cement chemistry and cement technology to control thealkali content in cement when such content is unfavorable. Alkalicombinations in the cement may appreciably shorten the cementsolidification time and bring about a breakdown or reversal of thecement. Too high an alkali content may lead to efflorescence of alkalisulfates in the concrete. Then, too, a high alkali content may result inreaction of the alkali material with additives to cause an alkaliexpansion of the cement and endanger the constancy of volume of theconcrete.

When pulverent raw material to be used in the manufacture of cement iscalcined, the alkali chlorides are volatilized quantitatively in thecalcining furnace and are conveyed off as vapors or by condensation as afine mist with the furnace exhaust gas. If the alkali materials in theexhaust gas reach the preheater, they are conveyed back into thecalcining furnace in combination with the heated raw material. Thefurnace exhaust gas is thereby constantly enriched with alkali and theraw material is consistently being increased in its alkali contentprogressively. Consequently, the pulverent raw material used in themanufacture of cement is appreciably impaired with regard to itsfusibility and it may result in a caking or clogging of material withinthe preheater.

In U.S. Pat. No. 3,235,239 there is described a method for the calciningof alkali-containing pulverent raw material to be used in themanufacture of cement in which the preheating of the pulverent rawmaterial takes place in a preheater with alkali-free exhaust air fromthe cooler in a separate combustion chamber. The alkalies volatilized inthe sintering furnace are partially displaced and partially conveyedinto the combustion chamber so that as the operating time increases, analkali circulation is built up in the installation. With very largecement production installations, however, it is not economical tocompletely displace or abandon the alkali-containing furnace exhaustgases without making use of their heat content since the cost of cementproduction particularly with present day high energy costs becomes veryhigh.

In German Pat. No. 1,471,115 there is described a method for theproduction of a low alkali cement from calcite or limestone containingpulverent raw material in which the raw material is calcined with analkali free exhaust gas from a cooler in a preheater during addition offuel and is sintered in a calcining furance. The alkali containingexhaust gases of the sintering furnace are cooled by means of preheatingof at least a portion of the raw material. The alkali salts condensethrough the cooling of the furnace exhaust gases and are removed in adust removal installation. With this method, a low alkali cement can beproduced from alkali containing pulverent raw material withoutsubstantial loss of heat, if it is possible to prevent a substantialprecipitation of alkali materials in the heat exchanger on the pulverentraw material. However, in practice it is difficult to accomplish thisresult with high alkali contents, particularly with direct heatexchange.

SUMMARY OF THE INVENTION

The present invention provides an improvement in the method for thethermal treatment of alkali-containing pulverent raw materials in themanufacture of cement, whereby the materials are treated with minimumheat losses and without danger of circulation of alkali. The equipmentinvolved can be provided at low investment costs. It is also possible toadjust the alkali content in the finished calcined produces despitefluctuating alkali contents in the raw material, to hold the contents atpredetermined permissible values.

In accordance with the present invention, a partial stream from thefurnace exhaust gases containing alkali being supplied to the heatexchanger is branched off before the heat exchanger and bypasses thesame. This partial stream is removed from the system after dust isremoved from it. With this arrangement, despite changing alkali contentsin a starting material, an approximately constant alkali content will beachieved in the cement clinker, as the undesired high quantity of alkalivolatilized in the sintering furnace may be conveyed out of the systemand only that quantity of alkali-containing furnace exhaust gas issupplied to the heat exchanger which will provide the permissivepredetermined alkali content in the cement clinker. The heat losses perkilogram of calcined cement clinker may in this way be substantiallylowered.

In the preferred form of the invention, the alkali-containing furnaceexhaust gases are conveyed to the heat exchanger where they give offtheir heat to a pulverent raw material used in the manufacture of cementand present in suspended form. Through the direct contact of the hotfurnace exhaust gases with the pulverent raw material, a very good heattransfer from the furnace exhaust gases to the pulverent raw material isobtained. The alkali materials are strongly cooled in the heat exchangerby means of the pulverent raw materials so that they condense in thefurnace exhaust gas and may be drawn off out of the heat exchanger to asubstantial extent in the form of a mist at a temperature of about 800°to 900° C. The pulverent raw material because of the spontaneous heatintake is calcined and is drawn off out of the heat exchanger.

In a further embodiment of the invention, the partial stream of thealkali-containing furnace exhaust gases is cooled by means of fresh airand/or water and/or pulverent cement raw materail. By these means, arapid cooling of the hot partial gas stream is obtained so that thealkali salts condense and are stabilized by means of the cooling to beseparated in the dust removal installation. Caking of the material inthe partial stream branch conduit is completely prevented. By theaddition of finely divided pulverent raw material, the dust resistancevalues are favorably modified so that the electrical precipitator canoperate in the optimum range and a substantial dust removal is insured.

In the preferred embodiment of the invention, the amount of the partialstream and/or the portion of the pulverent raw material which isintroduced into the stream of furnace exhaust gas is so adjusted thatthe total quantity of alkali introduced into the sintering furnace withthe heated pulverent raw material remains below a predetermined limit.By the adjustment of the particular quantity relation of the alkalicontaining partial stream to the alkali containing stream of exhaust gasconveyed to the heat exchanger, it is possible even at variably highalkali contents to adjust to a constant low alkali content in the cementclinker when it is calcined. It is also possible to vary concurrentlywith the adjustment of the gas-quantity ratio or separately therefromthe quantity of pulverent raw material to be used in the manufacture ofthe cement to be conveyed to the heat exchanger so that despite risingalkali content in the furnace exhaust gas, less alkali material reachesthe sintering furnace.

In accordance with a preferred embodiment of the invention, thepulverent raw material to be used in the manufacture of cement passesdirectly from the heat exchanger into the sintering furnace. This isadvantageous particularly when the pulverent material when treated bythe furnace exhaust gas is calcined in the heat exchanger to such anextent that a return into the preheater is not economical. It is,however, also possible to use a preheater with several cyclone stepsarranged in superimposed relation and to introduce the pulverent rawmaterial drawn off the heat exchanger, together with the pulverent rawmaterial which is preheated in the preheater into the lowermost cyclonestep. This is particularly advantageous when the pulverent raw materialdrawn out of the heat exchanger is only partially calcined so that afurther calcination, together with the pulverent raw material suppliedto the preheater may be carried out in the lowermost cyclone step ifnecessary by means of separate fuel addition. The entire pulverent rawmaterial preheated in this manner then leaves the lowermost cyclone stepof the preheater into the sintering furnace.

In a further embodiment of the invention, at least a partial quantity ofthe preheated pulverent raw material is conveyed out of the next highercyclone into the discharge end of the heat exchanger, and the combinedpulverent raw material is conveyed to the lowermost cyclone step of thepreheater. Generally, the danger exists with a furnace exhaust gas veryhigh in alkali and a raw material high in alkali that adhesions anddeposits may occur in the discharge conduit of the heat exchanger. Inour invention, however, the mixing of the two streams of pulverent rawmaterial results in no adhesion nor deposit in the discharge conduit ofthe heat exchanger.

The invention also provides an apparatus for carrying out the method,including a sintering furnace, a preheater connected in series forreceiving the pulverent raw material to be used and a cooler forsintering material connected with the exhaust air side of the preheater.The sintering furnace is connected through an exhaust gas conduit with asupply device for a portion of the pulverent raw material and a heatexchanger and a dust removal installation. There is further provided abranch conduit from the exhaust gas conduit of the sintering furnace forthe heat exchanger which bypasses the heat exchanger and preferably goesto the dust removal installation such as an electrostatic precipitator.By means of this system, when the alkali content rises in the startingmaterial, a predetermined quantity of alkali containing furnace exhaustgas is supplied to the heat exchanger and a partial stream is displacedso that the quantity of alkali remaining in the system may be adjustedto predetermined limits. The discharge of the branch conduit preferablyoccurs in front of the supply device for the pulverent raw material sothat the added or supplied pulverent material comes in contact solelywith a partial quantity of alkali containing furnace exhaust gases.

In a further embodiment of the invention, there is provided a materialdistributor beneath the supply device for the pulverent raw material. Inaddition, the heat exchanger is a cyclone separator. This combinationprovides a particularly simple means for securing an optimum suspensionof pulverent raw material in the furnace exhaust gases, and an optimumheat transfer of the heat content of the gases to the particles. A highdegree of separation of pulverent raw material from thealkali-containing exhaust gases is insured. The furnace exhaust gasescoming out of the heat exchanger are preferably cooled directly upontheir exit from the heat exchanger so that the still mist-like alkaliespresent condense further and are separated as solids in the dust removaldevice. Only a small part of the alkalies collect on the finely dividedparticles of pulverent raw material and leave the cyclone separatorthrough its discharge conduit. In this way, it is possible to treat apulverent raw material with very high alkali content fuel gases, makinguse of the full heat content of such gases so that the preheatedpulverent material has a predetermined alkali content which isvolatilized quantitatively in the sintering furnace. A cement clinkerwith very low alkali content is thus produced. The dust forced out ofthe cyclone separator improves the dust resistance values, so thatdespite a high proportion of alkali dust in the exhaust gas, an optimumseparating effect can be obtained in the electrical dust removalinstallation.

In a further embodiment of the invention, adjustable valves are providedin the furnace exhaust gas conduit and in the branch conduit. Thus it ispossible to adjust the ratio of partial stream to the exhaust gasessupplied to the heat exchanger so that even with an increase in thealkali content of the starting material, higher alkali values need notappear in the cement clinker when calcined.

In another preferred embodiment of the invention, the furnace exhaustgas conduit and the branch conduit discharge directly out of the furnaceinlet manifold and in each case have separate locking members. This isan advantage when the inlet manifold is such that an uncontrolledquantity of spray material is being carried along by the furnace exhaustgases. In accordance with this invention, the branch conduit is arrangedso that the spray material reaches only into the furnace exhaust conduitleading to the heat exchanger and is conveyed back together with thesupplied pulverent raw material into the stream. An excessive occurrenceof dust for removal by the dust removal equipment is thus prevented.

Additional features of the invention will be explained in greater detailin the following description of preferred embodiments by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will be readilyapparent from the following description of certain preferred embodimentsthereof, taken in conjunction with the accompanying drawings, althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts of the disclosure, and in which:

FIG. 1 is a diagrammatic showing of an installation for cementproduction carrying out the method of the present invention with twoparallel suspension gas preheaters consisting of several cyclones;

FIG. 2 is a modified form of the system shown in FIG. 1; and

FIG. 3 shows a further modification of the system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The installations shown diagrammatically in FIGS. 1 to 3 consist ofpartial views for the thermal treatment of alkali containing pulverentraw material used in the manufacture of cement. The systems include arotary kiln 1 with an inlet manifold 2 and two parallel suspension gaspreheaters 4 consisting of several cyclones 3 arranged in superimposedrelation. In the uptake or feed pipe 5 leading to the lowermost cyclone3, IV of one of the suspension gas preheaters 4 is connected a coolerexhaust air conduit 6 which is connected with a sintered materialcooler, not shown in detail, for the pulverization of raw materials tobe used in the manufacture of cement burnt to completion in the rotarykiln 1. The cyclones 3 of one of each of the suspension gas preheaters 4are connected by means of gas conduits 7,8 with one another in such amanner that the hot gases of the lowermost cyclone 3, IV, by means of ablower (not shown) are directed through the individual cyclones of thesuspension gas preheater 4. On the material side, the cyclones 3 of thesuspension gas preheater 4 are so connected with one another that thepulverent raw materials delivered to the preheaters are supplied incountercurrent relationship to the rising hot gases at 9,10 in the gasconduits 5,7 leading to the cyclones 3, IV and 3, III. The flow ofmaterial and hot gases in the suspension gas preheaters is indicated bydifferent arrows. The reference characters 3, I to 3, IV likewiseillustrate only diagrammatically the material discharge from theparticular cyclone step. The pulverent raw material is supplied in theuptake or feed pipe at 9 leading to the cyclone 3, IV, and is calcinedin suspension by means of combustion devices 11. The dust discharge pipeof the cyclone 3, IV, is in each case connected with the furnace inletmanifold 2.

On the exhaust gas side, the rotary kiln 1 is connected through thefurnace inlet manifold 2 with a furnace exhaust gas conduit 12 which isdirected to two heat exchangers 13, the heat exchangers being cycloneseparators. These separators are, in turn, connected through a gasconduit 30 with a spray tower 14, an electrofilter 15 such as anelectrical precipitator, and an induced draft blower 16 connected inseries. In the portion of the gas conduit 30, leading from the cycloneseparator 13 to the spray tower 14 there are provided fresh air conduits37 which are supplied by a blower 38 and discharge directly behind thecyclone separators in the gas conduit. Directly adjacent to the exhaustgas conduit there is a branch conduit 17 which bypasses the heatexchanger 13 and is connected to the spray tower 14 and theelectrofilter 15.

The discharge conduits 26 of the heat exchanger are connected to thematerial discharge pipe 28 of the next higher cyclone separator 3, III,of the suspension gas preheaters 4 which in turn are connected to thelowermost cyclone steps 3, IV at reference numeral 9.

The furnace exhaust gas conduit 12 includes a supply device 18 forpulverent raw material to be used in the manufacture of cement, andbelow the supply device there is located a deflecting plate 19. In thebranch conduit 17 there is similarly a supply device 20 for pulverentraw material and a water spraying device 21. In the discharge conduit 12and in the branch conduit 17 extending from the manifold 2, there areprovided fresh air conduits 22 and 23 which are connected to fresh airblowers 24 and 25, respectively.

The installation shown in FIG. 2 differs from that shown in FIG. 1 inthat the discharge conduit 26 of the heat exchanger 13 is in connectionwith the discharge pipes 27 of the lowermost cyclones 3, IV, of thesuspension gas preheater 4.

The installation shown in FIG. 3 differs from that shown in FIG. 1 inthat the material discharge pipe 28 of the next higher cyclone step 3,III, of the suspension gas preheater 4 is connected to the dischargeside 29 of the heat exchanger 13 and the discharge conduit 26 of theheat exchanger discharges at 9 in the uptake or feedpipe 5 leading tothe lowermost cyclone 3, IV.

Through a series of blocking members of known construction, not shown indetail, in the material discharge pipe 28 of the cyclone step 3, III, ofthe suspension gas preheater as well as in the discharge conduits 26 ofthe heat exchanger, the material transfer shown in FIGS. 1 to 3 betweenthe suspension gas preheater 4, heat exchanger 13, and rotary kiln 1 maybe carried out in a single installation so that an optimum adjustment ofthe installation is possible in any type of operation.

In the gas conduit 30 leading to the spray tower 14 and in the branchconduit 17 there are regulating valves 31,32 for the adjustment of therelative amounts of exhaust gases in those conduits. Opposite thefurnace inlet manifold 2, the furnace exhaust gas conduit 12 and thebranch conduit 17 are provided with blocking members or baffles 33 and34.

The manner of operation of the installations is summarized as follows.The pulverent raw material is preheated in the individual cyclone steps3,I; 3,II; and 3,III; and then passes into the feed pipe 5 where it ismixed with alkali free exhaust air from a cooler exhaust conduit 6. Theraw material is subjected in the feed pipe with the aid of combustiondevices 11 to a separate calcining process and after a significantcalcination has been effected, it is separated from the lowermostcyclone step 3, IV, and introduced in the rotary kiln 1. In the kiln 1,the alkali contained in the pulverent raw material is volatilizedquantitatively and together with the furnace exhaust gas passes throughthe furnace exhaust conduit 12.

To economically recover the heat content of the furnace exhaust gases,they are supplied through the conduit 12 to the heat exchangers 13.Approximately 10 to 15% of the total pulverent raw material supplied tothe installation is introduced by means of a supply device 18 into theexhaust gas conduit 12 from a storage container 35. In order to obtaingood heat transfer, the pulverent raw material is suspended with the aidof the deflecting plate 19 in the exhaust gas conduit 12 and rises withthe exhaust gases to the cyclone heat exchangers 13. There, theparticles are separated from the gas and together with the pulverent rawmaterial, already preheated to a substantial extent, is introduced fromthe cyclones 3, III, of the gas preheater 4 into the lowermost cyclonestep 3, IV. The hot gases issuing from the heat exchangers 13 whichcontain substantial amounts of alkali present in mist form are cooledwith fresh air through a blower 38 and the fresh air conduit 37 so thatthe alkalies may be conveyed along in dust form by the gases to theelectrofilter 15. Agglomerates in the gas conduit 30, particularly onthe heat exchanger outlet, are prevented.

If the alkali content in the exhaust gases should increase because of anincreased alkali content in the starting materials, then a partialstream of the alkali containing furnace exhaust gases is taken offthrough the branch conduit 17 with the aid of the regulating valves 31and 32. This fraction is removed with bypassing of the heat exchangerand flows through the spray tower 14 and the electrofilter 15 whereuponit is removed from the system. To keep the branch conduit 17 free fromadherent alkali deposits, a fresh air blower introduces air through theconduit 23 so that the alkalies condense early and may be conveyed offas alkali salts with the furnace exhaust gases. The condensation of thealkalies may be enhanced by the introduction of atomized water which isintroduced through the spray device 21 in the branch conduit 17. It isalso advantageous to enhance the condensation of the alkalies in thegases of the branch conduit 17 by adding pulverent raw material so thatthe dust resistance of values in the particles are more favorable forseparation in the electrofilter 15. The enrichment of the furnaceexhaust gases with pulverent raw material may also take place throughthe adjustable reflectors 36 in the discharge pipes 27 to improve thedust resistance values.

The alkali containing furnace exhaust gases in the furnace gas conduit12 being supplied to the heat exchangers 13 may be cooled by means offresh air through the fresh air conduit 22 and the blower 24, therebyaccelerating condensation of alkali.

If it should be found that the particles of pulverent raw material arealready sufficiently calcined, and the additional heat treatment in thelowermost cyclone step 3, IV, of the preheater 4 becomes superfluous,then this portion of raw material can be supplied with the pulverent rawmaterial discharged from the step through the discharge pipe 27 directlyinto the rotary kiln as shown in FIG. 2. If, on the other hand, thealkali content in the furnace exhaust gases rises substantially becauseof the change of alkali content in the starting materials, and thedanger exists that the discharge conduit 26 of the heat exchanger willbe clogged because of deposited alkali, then the raw material for cementmanufacture drawn out of the next higher cyclone step 3, III, can bedirected into the discharge end 29 of the heat exchanger 13 and thetotal quantity of raw material conveyed through the discharge conduit 26for further thermal treatment in the lowermost cyclone step 3, IV, ofthe preheater 4 (FIG. 3).

The method described for the thermal treatment of raw material to beused in the manufacture of cement with a high alkali content is notlimited to the embodiment shown by way of example where two parallelsuspension gas preheaters are combined with centrally disposed heatexchangers, but the process can also be used with an installationcontaining only one suspension gas preheater and one or more correlatedheat exchangers.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

We claim as our invention:
 1. In a method for the thermal treatment ofalkali-containing raw material for the manufacture of cement in whichsaid raw material is preheated in a preheater, sintered in a furnace,and cooled in a material cooler and wherein an alkali-free exhaust fromsaid cooler is conveyed to said preheater for preheating said rawmaterial and alkali-containing exhaust gases from said furnace arecontacted with a portion of said raw material in a heat exchanger tocondense alkali salts thereon, the improvement which comprises:divertinga portion of said alkali-containing exhaust gases prior to said heatexchanger, removing dust from the diverted portion, and removing thedust-free portion from the system.
 2. A method according to claim 1 inwhich said portion is cooled prior to precipitating dust therefrom.
 3. Amethod according to claim 1 in which said alkali-containing exhaustgases suspend said portion of raw material in said heat exchanger.
 4. Amethod according to claim 1 in which the portion of raw materialcombined with the alkali-containing exhaust gases constitutes from 5 to25% of the total quantity of said raw material being treated.
 5. Amethod according to claim 1 which includes the step of cooling saidalkali-containing gases with fresh air before directing the same intosaid heat exchanger.
 6. A method according to claim 1 in which the gasesissuing from said heat exchanger are cooled directly as they issuetherefrom.
 7. A method according to claim 1 in which the solid materialissuing from said heat exchanger is passed directly into said sinteringzone.
 8. A method according to claim 1 in which the solid materialissuing from said heat exchanger together with the raw material treatedin said preheating zone are passed as a suspension through a sequentialseries of cyclone stages.
 9. A method according to claim 8 in which atleast a portion of the preheated raw material from one of theintermediate cyclone stages is passed into the discharge end of saidheat exchanger and the combined materials from said heat exchanger arepassed into the first cyclone stage.